Safe Query Parser

Parse one URL query string (no leading ?).

• Split on & and skip empty segments.
• Split each segment at the first =; with no =, the value is empty.
• Decode key and value: + becomes a space, then %XX (two hex digits, any case) becomes that byte. A % not followed by two hex digits stays as a literal %.
• Skip pairs whose decoded key is empty.

Print key=value for each pair in order, or EMPTY if there are none. All decoded bytes are printable ASCII.